Black cutworm (BCW) is one of many cutworm species that attack early vegetative stage corn across the Midwest each year. It is a migratory pest that arrives in crop fields each spring. Numbers vary depending on how many moths arrive from the south. The arriving moths lay eggs in and around crop fields. When the larvae hatch, they often migrate to corn seedlings and feed on leaf tissue or worse, cut the seedlings off at or below the soil level. The population of larvae can be very sporadic; therefore, it is very important to scout and determine if management is needed.
Black cutworm larvae are light grey to black, have a greasy sheen to their skin, and have four pairs of fleshy prolegs at the end of their abdomen.1 Pairs of dark tubercles, or bumps can be seen along the sides of the body (Figure 1). The tubercles are an important identifier for BCW because similar looking cutworm species, such as the dingy cutworm, rarely cause economic injury in corn. On each BCW body segment, the tubercle closest to the head is about 1/3 the size of the tubercle closest to the rear. Tubercles on a dingy cutworm are all roughly the same size.
An increased risk of BCW damage typically occurs in fields that are low lying, poorly drained, or have lots of weed growth. Increased risk of BCW damage also can occur in fields with near permanent vegetation, reduced tillage or no-till, and with cover crops. Green plant tissue is attractive to females laying eggs; therefore, late planted slow-growing corn plants are typically more vulnerable to larval feeding. Fields should be scouted every 7 to 10 days until corn plants reach V5 growth stage.
Scouting should consist of examining 10 corn plants in five different areas of the field to reach a representative pressure estimation.2 Look for plants that are wilting, exhibit leaf discoloration, and are cut or missing altogether (Figures 2 and 3). The BCW can bury or drag cut plants under the soil surface to continue feeding during the day. They are nocturnal or cloudy day feeders and will hide in the soil during the day.
It is important to dig around injured plants to find the larvae. While scouting, pay careful attention for indicators of other early season pests such as wireworms, white grubs, and the seedcorn maggots. The economic threshold for BCW in corn is 2 to 3% cut plants when larvae are less than 3/4-inch long and 5% cut plants when larvae are greater than 3/4-inch long in size.2 Input costs, final stand, and corn price can influence the threshold. If a rescue treatment is needed, scout for BCW directly before treatment to make sure the larvae are still present.
1 Purdue University Field Crops IPM. Black Cutworm. Purdue University https://extension.entm.purdue.edu/.
2 Dean, A. and Hodgson, E. 2020. Scouting recommendations for black cutworm. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/.